As a method of manufacturing a carbon fiber, a method is widely known in which a precursor fiber of a carbon fiber is subjected to a flame-resistant treatment and a carbonizing treatment.
As the flame-resistant treatment used to manufacture the carbon fiber, for example, a method is widely employed in which a heat treatment is performed on the precursor fiber inside a heat treatment chamber in an oxidization atmosphere by hot air.
Incidentally, since the flame-resistant treatment is a treatment that accompanies an exothermal reaction, it is important to suppress a variation in reaction by keeping the uniform heat transfer performance throughout the inside of the heat treatment chamber.
Therefore, according to JP 10-237723 A (Patent Document 1), a heat treatment chamber through which a precursor yarn passes is provided with a blowing port that blows hot air in a direction following a precursor yarn passage path, and a cross-sectional area Ss of the heat treatment chamber and a cross-sectional area Sf of the blowing port in a direction perpendicular to the precursor yarn passage path have a relation of Ss/Sf≦2, so that a satisfactory parallel stream is formed along the yarn passage path. Further, according to JP 2002-194627 A (Patent Document 2), there is disclosed a nozzle in which a hot air introduction region is formed by a guide vane, a porous plate, and a rectifying plate, dimensions of respective portions inside a heat treatment furnace are defined by a predetermined relation, and a uniform blowing wind speed distribution is set so that hot air blows in parallel to a yarn.
Furthermore, according to a flame-resistant furnace disclosed in JP 2008-144293 A (Patent Document 3), the flame-resistant furnace includes a gas introduction portion that causes an oxidizing atmosphere gas to blow in a direction substantially perpendicular to a running direction of a polyacrylonitrile-based fiber bundle and a gas discharge portion that is installed to face the gas introduction portion and discharges the oxidizing atmosphere gas, and the gas introduction portion is provided with a porous plate having an opening with a circle of a diameter of 10 mm in consideration of a blockage of a hot air nozzle.